A Science Critique of Aurora by Kim Stanley Robinson

byPaul GilsteronAugust 14, 2015

I haven’t yet read Kim Stanley Robinson’s new novel Aurora (Orbit, 2015), though it’s waiting on my Kindle. And a good thing, too, for this tale of a human expedition to Tau Ceti is turning out to be one of the most controversial books of the summer. The issues it explores are a touchstone for the widening debate about our future among the stars, if indeed there is to be one. Stephen Baxter does such a good job of introducing the issues and the authors of the essay below that I’ll leave that to him, but I do want to note that Baxter’s novel Ultima is just out (Roc, 2015) taking the interstellar tale begun in 2014’s Proxima in expansive new directions.

by Stephen Baxter, James Benford and Joseph Miller

‘Ever since they put us in this can, it’s been a case of get everything right or else everyone is dead . . .’ (Aurora Chapter 2)

This essay is a follow-up to a review of Kim Stanley Robinson’s new novel Aurora by Gregory Benford, which critically examines the case that Robinson makes in the book that ‘no starship voyage will work’ (Chapter 7) – at least if crewed by humans. This is a strong statement, and even if the case is made in fictional form it needs to be backed up by a powerful and consistent argument. Greg criticises Robinson’s book mostly on sociological, political and ethical grounds.

Here, to complement Greg’s analysis, we take a critical look at the science in the book. Is Robinson’s ship a plausible habitat for a centuries-long voyage? Could the propulsion systems function as described? Is the planetary threat encountered by the would-be colonists biologically plausible?

This entry is mainly the initiative of Jim Benford, well known to readers of this blog; Jim is President of Microwave Sciences based in Lafayette, California, and his interests include electromagnetic power beaming for space propulsion. Also contributing has been Joseph Miller, biologist and neuroscientist, previously of the University of Southern California Keck School of Medicine, now at the American University of the Caribbean School of Medicine, with a long-time interest in extraterrestrial life. As for myself, I’m a science fiction writer, part-time contributor to such technical projects as the BIS-initiated Project Icarus, and author of some interstellar fiction myself, such as Ark (2009). And as the full-time writer I’m the one who got the privilege of writing up our conversations. Thanks, guys!

I should start by saying that Stan Robinson has been on my own (very short) list of must-read writers for the last twenty-five years at least, and that Aurora is a key book, as with all Robinson’s work deeply researched and deeply felt. If you haven’t bought the book yet, do so now.

Basics

Aurora is a tale of a multigeneration starship mission to Tau Ceti. (Note that Robinson’s starship is unnamed; here I’ve referred to it as ‘the Ship’.) The Ship reaches its target, but when it proves impossible to colonise the worlds there, a remnant of the crew struggles back to Earth.

This review is an analysis of technical and science aspects of this mission, based solely on evidence in the novel’s text. Of course any errors or misreadings of Robinson’s text are our sole responsibility.

We’ll be making comparisons with two classic studies. The BIS’s Project Daedalus (1978) was a study of an uncrewed interstellar probe which used the same fusion-rocket technology as did Robinson’s Ship in its deceleration mode. Daedalus had initial mass 50,000t (tonnes) of fuel (30kt deuterium (D), and 20kt helium-3 (He3)), the dry mass of its two stages amounted to 2700t, the payload was 450t, and the exhaust velocity was about 3.3%c, with cruise velocity 0.12c (c being the velocity of light). The Daedalus propulsion system was used only for acceleration; it couldn’t decelerate, and so was a flyby mission at its target star. In Aurora the Ship uses its fusion rocket only to decelerate.

Meanwhile the ‘Stanford Torus’ space habitat design (Johnson, 1976) was a product of a 1975 workshop involving NASA Ames and Stanford University. The final design was a torus 1790m across with the habitable tube 130m in diameter. Of a total surface area of about 2.3km2, 10,000 people would inhabit a usable surface area of about 0.7 km2. The station, located at L5, would be built of lunar resources. The total mass would be about 10 million t, of which 9.9 million t would be a radiation shield of lunar slag around the habitable ring in a layer 1.7m thick, leaving 0.1 million t as structural mass. The relevance to Aurora is that the Ship looks like two Stanford Toruses attached to a central spine.

Let’s begin by looking at the Ship’s construction and inhabitants.

The Ship

Construction

Most of what we learn about the Ship’s structure is given in Chapter 2. The Ship consists of a central spine 10km long, around which 2 rings of habitable ‘biomes’ spin, torus-like. Each ring consists of 12 cylindrical biomes, each 4km long, 1km diameter. There are also spokes and inner rings. The rings rotate around the spine to give a centrifugal gravity of 0.83g.

The 24 biomes contain samples of ecospheres from 12 climatic zones: Old World versions in one ring, New World in the other. Each biome has a ‘roof’ with a sunline, which models the required sunlight and seasonality, and a ‘floor’ on the side away from the spine. The liveable area in each cylinder is given as about 4 km2, which is about a third of the cylinder’s inner surface area: 96km2 total. In each biome there are stores under the ‘floor’, including fuel; we’re told this is used as a radiation shield during the cruise.

The crew numbers given appear contradictory; in some places Robinson states there are about 2100 total, but elsewhere is given a number of 300 people per biome which would total 7200. The crew numbers do vary through the centuries-long mission, with births and deaths.

How reasonable are these numbers, given the mission’s objectives? Could the Ship support that many people? Are they enough to found a human population at the target? And is there room for true wilderness?

Closed Ecologies

We don’t yet know how to maintain closed ecologies for long periods. The Ship’s biomes would suffer from small-closed-loop-ecology buffering problems, as Robinson illustrates very well in the text; we see the crew having to micro-manage the biospheres, and dealing with such problems as the depletion of key trace elements through unexpected chemical reactions. In some ways this may prove to be an even more daunting obstacle to interstellar exploration than propulsion systems.

Human population

If there are 300 people per biome, and given a total of 96km2 habitable area, that’s a population density of 75 /km2. Compare this with Earth’s global average of 13 /km2 ; crowded southern England is 667 / km2. In terms of the ability of the agricultural space (70% of total) to support the crew, that seems reasonable to us.

But if only 5% of the space is used for residential purposes, the effective living density is high, at 1500 per km2 – comparable to densely populated urban areas such as Hong Kong. Such densities would seem problematic on a long-duration mission, though of course the crew do have access to the other 95% of the habitable areas; people hike the wildernesses.

This group is of course meant to be sufficient to found a new human breeding population on a virgin world. What is the minimal population size to maintain the species without an evolutionary bottleneck? Something like 1000 is a good guess. Robinson’s original population was at least twice that. If that population size was maintained, genetic diversity would plausibly be sufficient.

‘Wilderness’

We’re told (Chapter 2) that each biome has about 4km2 of living space and that 13% of that space is given over to ‘wilderness’, that is 0.52 km2 per biome. The ecologies can include apex predators. In a biome called Labrador, for instance, ‘In the flanking hills sometimes a wolf pack was glimpsed, or bears’ (chapter 2).

This idea is explored in more depth in Robinson’s 2312, in which mobile habitats called ‘terraria’, hollowed-out asteroids, are used as reserves for species threatened on a post-climate-change Earth. But even these terraria are not very large in terms of the space needed by wildlife in nature. A wolf pack, consisting of about 10 animals, may have a territory of 35 km2 (Jędrzejewski et al, 2007). A 2312 terrarium with an inner surface area of about 160 km2 would have room for only about 4 packs, or about 40 individual animals, a small population in terms of genetic diversity.

It seems clear that the much smaller biomes of the Ship, though large in engineering terms, would be far too small to be able to host meaningful numbers of many animal species in anything resembling a natural population distribution. A wilderness needs a lot of room.

Mass

We are given a mass breakdown for the Ship as a whole. We’re told that during the Ship’s cruise phase, when it is fully laden with fuel, the total mass is 76% fuel, 10% each biome ring, and 4% the spine.

We aren’t told the Ship’s total mass, however, and to study the propulsion system’s performance we’ll need at least a guesstimate. This is derived by a comparison with the Stanford Torus design.

Each torus-like biome ring consists of 12 pods of length 4km, diameter 1km. So the surface area of 1 pod is 14.1 km2, including end caps. And the surface area of one biome ring is 170 km2 (which is much larger than the Stanford Torus).

The Ship’s biomes seem to lack a Stanford-like cloak of radiation-shielding material. Robinson says that ‘fuel, water and other supplies’ are stored under the biome floors to provide shielding; the ceilings are shielded by the presence of the spine. Elsewhere Robinson says that during the voyage, the fuel is ‘deployed as cladding around the toruses and the spine’ (Chapter 2)

Assume then that if a Ship biome ring has the same structural properties as the Stanford torus, and if most of its mass is in the hull, then a guesstimate for a single ring mass (without the fuel cladding) can be obtained by multiplying Stanford’s 0.1m tons structure mass (without shielding) by a factor to allow for the Ship ring’s larger surface area. The result is (0.1 * 170 / 2.3 =) 7.4 million tons per biome ring. We know this is 10% of the Ship’s total mass, which therefore breaks down as

These numbers shouldn’t be taken seriously, of course, except as an order of magnitude guide. Maybe they seem large – but remember that Daedalus needed 50,000t of fuel to send a 450t payload on a flyby mission to the stars, a payload comparable to the completed mass of the ISS. By comparison the Ship will be hauling two habitat rings each fifteen kilometres across. This is not a modest design.

And notice that since this fuel is used for deceleration only, the acceleration systems need to push all this mass up to ten per cent of lightspeed. These numbers do illustrate the monstrous challenges of interstellar travel, with a need to send very large masses to very large velocities, and decelerate them again.

On that note, let’s consider the propulsion systems.

Propulsion

Mission Profile

The Ship is a generation starship. Launched in 2545, it travels 11.8ly (light years) to Tau Ceti at cruise 0.1c (chapter 2). According to the text the journey consists of a number of phases.

The Ship is accelerated to the cruise speed of 0.1c by means of electromagnetic ‘scissors’ slingshot at Titan, imposing a brief’ acceleration of about 10g, and then a laser impulse for 60 years.

The Ship decelerates at the Tau Ceti system using its on-board fusion propulsion system. The technology, like that used by Daedalus, is known as ‘inertial confinement fusion’ (ICF), in which pellets of fuel are compressed, perhaps with laser or electron beams, until they undergo fusion; the high-speed products provide a rocket exhaust. For twenty years the Ship is decelerated by the detonation of fusion pellets at a rate of two per second. The fusion fuel is a mix of D and He3, as was the case for Daedalus (Chapter 1).

We’re told that the total journey time is about 170 years (Chapter 3), consistent with the profile given.

Colonisation in the Tau Ceti system is attempted and fails (this will be considered below).

A section of the crew chooses to return to the Solar System. The ICF system is refuelled at Tau Ceti, and used to accelerate the Ship to 0.1c (Chapter 5).

As the Ship’s systems break down, the surviving crew completes the final leg of the journey in cryosleep.

The Ship has no onboard way to decelerate at the Solar System (Chapter 6). The ICF fuel was exhausted by the acceleration from Tau Ceti, save for a trickle to be used during Oberth Manoeuvres (see below). The laser system reduces the Ship’s speed, but not to rest: from 10%c to 3%c. We’re told that the Ship then sheds the rest of this velocity mostly with 28 Oberth Manoeuvres, using the gravity wells of the sun, Jupiter, and other bodies. This process takes 12 years before crew shuttles are finally returned to Earth.

We can consider these phases in turn.

Acceleration from Solar System

In considering the acceleration system, it should be borne in mind that what we need to do is to give a very large, fuel-laden Ship sufficient kinetic energy for it to cruise at 0.1c. And because of inevitable inefficiencies, the energy input to any acceleration system will have to be that much greater.

In fact the launch out of the Solar System is a combination of two methods, vaguely described, neither of which is remotely efficient. There’s a ‘magnetic scissor’ that accelerates the ship over 200 million miles: ‘…two strong magnetic fields held the ship between them, and when the fields were brought together, the ship was briefly projected at an accelerative force equivalent to 10 g’s’.

(Of course such acceleration would stress the crew, even though in tests humans have survived such accelerations for very short periods – indeed the book claims five crew died. And such acceleration could stress lateral structures, such as the spars to the biome rings. Perhaps the stack is launched with its major masses in line with the thrust, and reassembled later.)

In Jim Benford’s grad school days, he ran some actual experiments on this effect, using a single turn coil. The energy in the capacitor bank driving it was about 1 kJ and the subject of the acceleration was a screwdriver sitting on a piece of wood in the coil centre. The coil current pulsed to peak in 2 µs. The screwdriver was accelerated across the room to a target at about 10 meters per second. The kinetic energy of the screwdriver was about 5 J and therefore the efficiency of transfer was less than 1%. It seems unsafe to assume an efficiency much better than this.

For the Ship, there then follows a laser driven acceleration. While lasers can certainly accelerate light craft, as has been shown experimentally, they can’t accelerate the enormously massive vehicle that the novel describes. The power required to accelerate by reflection of the laser photons can be calculated from the Ship mass (74 million tons), final velocity and acceleration time (to 0.1c in 60 years, so 0.17% g). The amount of power is about 100,000 TW, a truly astronomical scale. (Earth’s present electrical power output is 18 TW.) The efficiency of power beaming is low because only momentum is transferred from the photons to the ship. Efficiency is the time-averaged ratio of velocity to the speed of light. Therefore the efficiency of this process is about 5%.

The Ship and its mission would have to be a project of a very wealthy and very powerful interplanetary civilisation. It seems unlikely that they would resort to such a hopelessly inefficient system, if it could be made to work at all.

Deceleration at Tau Ceti

The Ship uses its onboard fusion rocket to decelerate.

We’re told the ICF deceleration phase takes 20 years at 0.005g, starting from 10%c cruise speed, with a Ship with an initial fuel load of 76% total mass. These numbers enable us immediately to calculate one critical number, the exhaust velocity of the fusion rocket. A ship with 76% fuel mass has a mass ratio (wet mass / dry mass) of (100/24=) 4.17. The rocket equation tells us that given that mass ratio and a total velocity change of 0.1c, the exhaust velocity must be 7%c. This is twice that of Daedalus, but perhaps not impossible for an advanced ICF system.

Our mass guesstimate above allows us to assess the performance of the rocket. Consuming 56.2mt of fuel in 20 years gives a mass usage rate of 94 kg/sec (cf Daedalus first stage 0.8 kg/sec). (Notice that the two fusion ‘pellets’ consumed per second are pretty massive beasts; in the Daedalus design pellets a few millimetres across were delivered at a rate of hundreds per second. This detail may be implausible. Indeed 49kg may be larger than fission critical mass!)

You can find the rocket’s thrust by multiplying mass usage by exhaust velocity, to get about 2000 MN (megaNewtons). This is much larger than the Daedalus first stage’s 8 MN. And the rocket power is 20,000 TW (the Daedalus first stage delivered 30 TW). Note that this power number is comparable to the launch figures.

Again, these numbers can be taken only as a guide. But you can see that the power generated needs to be maybe three orders of magnitude better than Daedalus, and exceeds our modern global usage by four orders of magnitude.

Meanwhile this system would consume a heck of a lot of fusion fuel. Where would you acquire that fuel, and where would you store it?

The storage is the easy part, relatively. Daedalus’s 50 kt of fuel was stored in six spherical cryogenic tanks with total volume 76,000 m3. At similar densities to store the Ship’s fuel load would require 860 million m3. That sounds a lot, but the volume of a biome ring is about 38 billion m3, so the fuel volume is only 2% of this, making it plausible that it could be stored, as Robinson says, in cladding tanks on the biome rings and spine, without requiring large separate structures. The Ship is big but hollow. It’s not immediately clear however how effective a layer of fuel would be as a cosmic radiation shield.

And note that the need for cryogenic store over centuries before use would be a challenge – as would the need to store any short-half-life propulsion components such as tritium, which has a half-life of 12.3 years, and would decay away long before the 170-year mission was over.
Getting hold of the fusion fuel, meanwhile, is the tricky part. It’s hard to overstate the scarcity of He3 in the Solar System, and presumably at Tau Ceti. Even Daedalus’s 20,000t would deplete the entire inventory of the isotope on Earth (37,000t), and the Ship’s 22.5mt would dwarf the Moon’s store (1 million t); only the gas giants could reasonably meet this demand (the Daedalus estimate was that the Jovian atmosphere contains about 1016 t). The Daedalus design posited acquisition from Jupiter, but estimated that to acquire Daedalus’s fuel load in 20 years would require that the Jovian atmosphere be processed at a rate of 28 tonnes per second. So again the challenge for the Ship’s engineers will be three orders of magnitude more difficult.

And regarding the return journey, although the Ship is stripped down, a fuel load of similar order of magnitude must be acquired from the Tau Ceti system, and without the assistance of a Solar-System-wide infrastructure. Of this huge project, Robinson says only that ‘volatiles came from the gas giants’ (Chapter 4).

Deceleration at Solar System

At the end of the novel, the Ship returns to Earth, decelerating mostly using what is called the ‘Oberth Manoeuvre’, invented by Hermann Oberth in 1928. This is a two-burn orbital manoeuvre that would, on the first burn, drop an orbiting spacecraft down into a central body’s gravity well, followed by a second burn deep in the well, to accelerate the spacecraft to escape the gravity well. A ship can gain energy by firing its engines to accelerate at the periapsis of its elliptical path.

Robinson wants to use this to decelerate from 3% of light speed down to Earth orbital velocity. 3% of lightspeed is 9,000 km/s. For reference, Earth’s orbital velocity is 30 km/s. Several deceleration mechanisms are referred to in the book. An unpowered gravity assist, passing by the sun and reversing direction, can steal energy from the sun’s rotational motion around the centre of the galaxy. That’s worth about 440 km/s. Other unpowered gravity assists can be used once the ship is in a closed orbit in the sun’s gravitational well. Flybys for aerobraking in the atmospheres of the gas giants are referred to as well. Altogether, these can get you <100 km/s.

But the key problem with using the Oberth Manoeuvre for deceleration of this returning starship is that this craft is on an unbound orbit. That means that, on entering the Solar System its trajectory can be bent by the sun’s gravity, but will then exit the System because it has not lost enough velocity to be bound to the Solar System. To be bound would require velocity decreased down to perhaps 100 km/sec, which is 1% of the incoming velocity. Therefore 99% of the deceleration has to take place in the first pass. And you can’t get that much from an Oberth Manoeuvre.

Cryosleep

As the Ship’s systems collapse, the returning crew gets from Earth plans to build a cryonic cold sleep method, which allows the viewpoint characters to survive until they reach the Earth.

This technology logically undermines most of the problems the early parts of the novel confront, and therefore undermines most of Robinson’s point about the difficulty of interstellar travel: If only the colonists had waited a few centuries for cryo technology, it would all have been so much easier! But this contradicts Robinson’s thesis.

Aurora

Having arrived at Tau Ceti, the colonists’ target planet, called Aurora, is judged lifeless but habitable from a remote sensing of an oxygen atmosphere – presumed created by non-biological process billions of years ago – but in the event the environment proves lethal for humans because of the presence of a deadly ‘prion’.

In a sense this is the point of the novel, that even if we reach the stars we will find only dead or hostile worlds: ‘I mean, they [alien worlds] are all going to be dead or alive, right? If they’ve got water and orbit in the habitable zone, they’ll be alive. Alive and poisonous . . . What’s funny is anyone thinking it [interstellar colonisation] would work in the first place’ (chapter 3). And as Greg noted in his essay this reflects recent misgivings expressed by Paul Davies and others about the habitability by Earth life of exoplanets.

Is this reasonable? And is Robinson correct that this could be the solution to Fermi’s famous paradox?

Robinson seems to be saying ‘alive’ worlds will be toxic to all possible biological explorers (there is a little wiggle room here since non-biological automated probes might still survive such worlds). This is a bold statement, but plausible since we lack any relevant data. However Robinson also says ‘dead’ worlds, essentially rocky Earth-size planets in the Goldilocks zone, could be terraformed but that project would take thousands of years. But why should that matter in a galaxy that is billions of years old? There should be plenty of time to terraform such planets, either by biological explorers or perhaps some type of self-replicating von Neumann probes or seed ships. There appears to be no solution in Aurora to Fermi’s question.

Oxygen and Biosignatures

(See Sinclair et al (2012) for a relevant reference.)

It seems implausible that oxygen in Aurora’s atmosphere might not be a biosignature: that is, that it could credibly be created by non-biological processes. Without some continual input into the atmosphere, you would expect any oxygen to rust out, as on Mars. Robinson says the oxygen on Aurora is due to the ultraviolet breakdown of water. We haven’t run the numbers, but that would be a hell of a lot of UV (which itself could make the planet uninhabitable). That might actually work better as a mechanism for oxygen production on Mars, at least long ago when Mars had liquid water. Indeed, UV is how Mars lost its water and atmosphere, and the same would happen on a dead Earthlike world. So Aurora can’t have oxygen; it gets blown off after the hydrogen from water.

Robinson also cites a failure to detect CH4 and H2S, possible markers of life, in Aurora’s air as ruling out a biological origin for the oxygen. However the interpretation of the presence of methane (CH4) in the Martian atmosphere has been a bone of contention for well over 15 years. Is it a biomarker or an index of geological activity? And as far as hydrogen sulphide goes, it sure as hell is not a biomarker on Io!

The ‘Prion’

The most significant biological problem in Robinson’s scenario is the organism that was so toxic to humans on Aurora. This is said to be ‘something like a prion’, and is apparently an isolated organism: as far as the explorers could tell there simply was no wider biosphere on Aurora.

For a biologist, that sounds really weird. This is a satellite a couple of billion years older than Earth and the only evolved organism is a prion? In addition we are not sure what ‘something like’ really means, but if it was indeed like a prion one must ask: where on Aurora are the proteins capable of being misfolded by a prion action? That’s what prions do; they cannot exist in isolation. And then why was it that human proteins, from a different biosphere altogether, were such a good match to the prion’s mechanisms?

Of course you can say it was ‘something like’ a prion but not really a prion. But then, what makes it ‘like’ a prion if not protein-folding?

It would take a lot more detail to make this strange single-organism biosphere a plausible ecosystem. Maybe if Robinson ever revisits Aurora and the stayers we could find out! Joe Miller thinks that an Andromeda Strain-like organism, inimical to Earth biology, is no more or less likely than ET organisms which simply find Earth biology indigestible. We don’t know, but the possibility that ET biology would be simply oblivious to Earth biology is a plausible situation, though not treated very much in SF because it is not very dramatic!

Conclusions

Robinson’s Aurora is a finely crafted tale of human drama and interstellar exploration. Its polemic purpose appears to be to demonstrate, in Robinson’s words, that ‘no [human-crewed] starship voyage will work’. There is much of the science and technology we haven’t explored in this brief note; there’s probably a master’s thesis here – indeed I’ve recommended the book to Project Icarus as a study project.

However, to summarise our conclusions:

The human crew transported to Aurora may plausibly be large enough to support a new breeding population. And the Ship’s dimensions seem adequate to support the crew through their centuries-long mission.

The challenge of maintaining small closed biospheres is depicted credibly, but the ‘wilderness’ areas of the biome arks are too small for their purpose.

Of the elements of the propulsion system, the electromagnetic / laser Solar System acceleration system needs to be so powerful it stretches credibility, while the Oberth Manoeuvre return-deceleration system as depicted is impossible. The ICF fusion rocket system appears generally credible, but would require the acquisition of heroic amounts of helium-3 fuel, a challenge especially at Tau Ceti.

Regarding Aurora itself, the notions of a non-biogenic oxygen atmosphere, and of a single-organism biosphere, and that an extraterrestrial organism as described might necessarily be inimical to humans, all lack credibility.

In summary, while Aurora is an intriguing combination of literary, political, scientific and technical notions, and while it reflects many current speculations about the difficulty of interstellar travel, in many instances it lacks the supporting credible scientific and technical detail required to make its polemic case that human interstellar travel is impossible. The journey is not plausible, and nor is the destination.

What Aurora illustrates very well, however, at least at an impressionistic level, is the tremendous difficulty of mounting such a voyage. Interstellar travel is a challenge for future generations, which will bring both triumph and tragedy.

Joy August 15, 2015 at 3:06 reminded us of a scenario where robots are the critical component . Perhaps this could work , but it could also be most dangerous mistake ever made . Such robots would nescesarrilly be capable of maintaining themselves and everything else on the ship for a very long period , and so they would have to be capable of some form of re-producing themselves , like social insects .
Some of the most talented people of our generation ( including Stephen Hawking and Ellon Musk ) believes we have to extremely careful NOT to produce a mecannical lifeform capable of reproducing itself , and a generationship is exactly the place for this mistake to breed a long term disaster that could make nuclear war look like a picnic .
Perhaps this danger can be avoided safely somwhere out in the future , but from the perspective of 2015 , for us to be playing with completely autonomous and selfreplicating robots in a generationship , is like a 17’th century engineer trying to deactivate a stockpile of hydrogenbombs programmed to go off if anybody try to deativate them …
Better plan for a scenario which does not gamble with the future of our species .

Ole: “Evolution is the most complicated information-processing system in the known universe . We can learn from it how to change ourselves just enough to fit the jobdescription in a generationship , and other space habitats .”

Complexity is not equivalent to effectiveness or efficiency. Evolution is akin to a government bureaucracy: it usually get the job done, though almost never in an optimum fashion! While we can learn from the things evolution has chanced upon, engineering is more promising for human endeavors.

You might want to check out http://www.spaceref.com astrobiology web right now for a very interesting post on detecting light sail leakage. Artificially produced microwaves PROPELLING a lightsail from one planet to another in a solar stem inhabited by technologically advanced civilization will be EASILLY DETECTABLE, but for only a few seconds, and will not repeat at regular intervals, BUT, a DEDICATED search lasting DECADES after the first CANDIDATE signal has been detected, could lead to enough IRREGULAR repetes to eliminate ALL OTHER POSSIBLE CAUSES, ESPECIALLY if the frequency range falls between two magic frequencies (EXAMPLE: Pi TIMES Hydrogen, and Pi PLUS Hydrogen vary by olny about 10 megahertz. TYC 1220-91-1 anyone

I’m not sure I agree with KSR’s implicit point at the end, which is that interstellar colonization by humans may be impossible because a human population simply won’t survive long enough without “island devolution”. The development of long-term hibernation opens up some new possibilities, namely that you could send their highly capable AI ahead of time to new solar systems to begin the terraforming process.

Make the world habitable enough so that the human colonists can survive on it and complete the terraforming process, and you have a successful colony. If the process fails while they’re in transit, then send them with the capabilities to return home if necessary.

I do think KSR is right about our goals potentially shifting by the time we have these capabilities. Most of the arguments that we must colonize space to survive depend highly on either extremely rare calamities or extremely long time periods. The few that don’t are man-made ones that may or may not be stuff we can resolve here on Earth (like bio-weapons).

The most ”obvious problems” facing humans in the cheapest-possible generationship are radiation-resistance , psycology and behaviour If these can be solved by genetic modification together with an advanced selectionprocess , the constraints of design will change dramaticly to a point where a low -mas ship becomes possible … much less than one % of ‘Aurora’

Those are a few problems, but evolution as you suggest will not solve teh other problems – teh mass of teh ship simply due to teh space needed, the ecosystem that must also survive, including your microbiome, the longevity issue and the need for multiple generations, teh moral issue of having children in such a situation, the issue of needing a habitable world that has oxygen but best if no extant life exists to disrupt or be disrupted by.

By eliminating the biological requirements as much as possible, we gain a virtuous circle of reduced mass and cost. Ideally we want to get as close to a “seed” as possible, so that a small mass can be propelled to the stars, in fact many of them . So we want the equivalent of an r-strategy that will allow us to attempt to seed many stars and where losses are not thinking humans – just stored information/embryos and associated support systems.

If we had a few billion years to play with, we could just send out frozen spores or seeds and let the successful evolve on their target worlds for us to visit in the far future. But well before then we should have the technology to travel space and live as artificial beings to explore and perhaps colonize the stars.

I started reading “The Martian” and give up in the 3rd page, after reading several big scientific mistakes (hey, a Martian storms can’t never be that strong!). This book seems to have similarly big mistakes, putting both works in the “fiction” category, instead of the “science-fiction” category. I wonder why this author is so popular.

The primary challenge of rocket propulsion is the burden of needing to accelerate the spacecraft’s own fuel, resulting in only a logarithmic gain in maximum speed as propellant is added to the spacecraft. Light sails offer an attractive alternative in which fuel is not carried by the spacecraft, with acceleration being provided by an external source of light. By artificially illuminating the spacecraft with beamed radiation, speeds are only limited by the area of the sail, heat resistance of its material, and power use of the accelerating apparatus.

In this paper, we show that leakage from a light sail propulsion apparatus in operation around a solar system analogue would be detectable. To demonstrate this, we model the launch and arrival of a microwave beam-driven light sail constructed for transit between planets in orbit around a single star, and find an optimal beam frequency on the order of tens of GHz. Leakage from these beams yields transients with flux densities of 0.1 Jy and durations of seconds at 100 pc. Because most travel within a planetary system would be conducted between the habitable worlds within that system, multiply-transiting exoplanetary systems offer the greatest chance of detection, especially when the planets are in projected conjunction as viewed from Earth.

If interplanetary travel via beam-driven light sails is commonly employed in our galaxy, this activity could be revealed by radio follow-up of nearby transiting exoplanetary systems. The expected signal properties define a new strategy in the search for extraterrestrial intelligence (SETI).

This is something that is not often pointed out regarding artificial
human habitats. A rotating wheel generating centripetal force is a good
substitute, on paper. No study that I know has experienced it even short
term let alone for decades. Are we missing subtle physiological surprises from substitute gravity. Does reproduction/development go without a hitch? There are a lot reassurances out there about it.
There was another theoretical engineering/physical construct
that should have worked according to theory and modeling.
Many expected the National Ignition Facility to create a quantum
leap in fusion generation.
I don’t think a large surprise like that awaits artificial gravity engineering
attempts. But since there are not current experiments we can point to, one
cannot rule out small unexpected effects that only show up after long term habitation of artificial gravity structures.

A generation ship would re-institute the practice of children being raised by parents, grandparents, aunts, cousins….whole families would be families again. The children would benefit in the same way these types of extended families work here. Maybe we should do an experiment akin to Mars500

RobFlores: “A rotating wheel generating centripetal force is a good
substitute, on paper. No study that I know has experienced it even short
term let alone for decades. Are we missing subtle physiological surprises from substitute gravity.”

For one answer we need only look at EEP (Einstein Equivalence Principle). This is at the core of general relativity, though we don’t need that to understand what EEP is about. Namely that, to an observer, constant acceleration (e.g. a rocket in space) is indistinguishable from gravitation (e.g. standing on Earth).

However, an observer on the inside of a wheel is not under constant acceleration. But it’ll be close enough if the wheel is large. The difference becomes clear if you jump high into the air or (easier) throw a ball on the rotating wheel. The trajectory will not be what you might expect. The smaller the wheel the greater the effect.

Some of the most talented people of our generation ( including Stephen Hawking and Ellon Musk ) believes we have to extremely careful NOT to produce a mecannical lifeform capable of reproducing itself

I think there is a misunderstanding here. The smart people you mention have warned against possible consequences of developing human level AI, not self-replicating machines. The distinction is important. You can make dumb self-replicating machines, which would be highly useful and perfectly harmless. They would never know when you pull the plug on them.

Human level AI is a different animal. Those would be able to outsmart humans. That includes keeping humans from pulling the plug, and defeating them in war. Even a perfectly contained and isolated AI would presumably be able to talk itself out of its box, eventually. This is scary stuff.

” A rotating wheel generating centripetal force is a good
substitute, on paper. No study that I know has experienced it even short
term let alone for decades. ”

My biggest beef about the space program. The need for a rotating space station to test this, and the minimum amount of gravity needed for long term health, has been an obvious need for most of my life. We literally do not know if lunar gravity is enough, Martian gravity is enough. For all we know, gravity levels greater than 1 G are actually healthier than normal Earth gravity! (They are for mice…)

AFAIK dissociation and loss with solar wind is the current understanding for hydrogen scarcity in Venus, so it only remains to explain why there wasn’t enough stuff (like carbon and iron) for the freed oxygen to react with…

NASA and other space agencies are way behind when it comes to knowing if humans can survive in space or on other worlds for long periods of time. And I mean long. They are still operating under the Right Stuff attitude from the early days when a few select men endured a few hours to a few months in a tin can.

It is one thing when trouble happens and you are circling Earth, where rescue or a trip home can be a matter of minutes away. What happens when you are halfway to Mars and your home planet is just a blue star in the sky?

There has been a lot of focus on the engineering details of spaceflight and space colonization. When it comes to those who will do the actual living in the Final Frontier, the attitude seems to be they will learn to manage – the equivalent of telling some injured kid to “walk it off” or “man up”.

Perhaps if more folks studied the people who have lived in the polar regions of Earth and what prolonged isolation and constant danger have done to many of them, they will not be so cavalier when it comes time to send our children into the void.

Oh, and just to make sure, this isn’t a lack of will or cowardice or being anti-space. This is making sure if we really do want to send humans to the stars that they actually get there in one piece to complete the missions we assigned them. Real space is not Star Trek.

The equivalence principle in a rotating ring, applies to a STATIONARY
body, inside the ring.

When travelling at angle that closer to the perpendicular of the direction of travel, there are other effects. Also the force differential you feel at the bottom as opposed to the top of your body is much more significant than
on the Earth’s surface, unless your wheel if very large.

Ron S : If you think Evolution works in ways similar to government bureaucracies , that would make me and you a result of a bureaucratic process …. including our loves , our hopes , our ability to rise above our selves in difficult situations …..Perhaps your bureaucracy in the US is a lot better than ours here in Israel !

Alex Tolley ”By eliminating the biological requirements as much as possible, we gain a virtuous circle of reduced mass and cost. Ideally we want to get as close to a “seed” as possible, so that a small mass can be propelled to the stars, in fact many of them”
About this we are in perfect agreement , but for me the closest we can get to a ‘seed’ is a very smal group of slightly modified people capable of surviving and feeling at home in a closed environment . To get any closer than that ,we would have to gamble on advanced robotic systems controlled by a human level AI , which COULD make the system as a whole capable of reproducing itself….more dangerous than all the nukes in the world .
The other possiblity is to give up on humans ….not on my watch !
What I tried to describe was ASSISTED evolution , an advanced selectionprocess for psycology and behaviourproblems , and genetic engineering for radiationressistance .
In another discussion here , we talked about lacerpropelled sails . The size of the lacerunits and their energy supply can be drasticly reduced if our starship can be accelerated while divided into many smaller units ( some of them with creew menbers abourd) , which are then assembled when the accelerationfase is over . The re-asemby would be a very difficult process , innvolving a lot of high-level dicisions and unexpected problems , just the stuff humans are best at .

Slightly off topic it is interesting that from this interactive animation another star Luyten 726-8 (A,B) is almost in line with Tau Ceti and another star YZ Ceti slightly off to the side of Tau, one probe could theoretically reach all three if ejection of probes towards them was managed at the right time. Just highlight Tau Ceti and rotate it into view with the Sun in the middle, neat little tool, you see another close line up near Procyon.

Eniac : ”You can make dumb self-replicating machines, which would be highly useful and perfectly harmless. They would never know when you pull the plug on them.’ ‘
You are right , there is some kind of misunderstanding . We were talking about the possibility of Human-like robots (perhaps controlled by a single human level AI ) who would be the ” Crew” of a starship delivering millions of frozen embryoes to the target star . They most be able to control and maintain the whole super-complex system autonomously, and deal with any unexpected problems . Later they most be capable of raising the first generation of humans , without causing the children too many psycologic problems ….while also in this area able to deal with unexpected problems .
Not really a case of ‘dumb selfreplicating mashines’….

‘In another discussion here , we talked about lacerpropelled sails . The size of the lacerunits and their energy supply can be drasticly reduced if our starship can be accelerated while divided into many smaller units ( some of them with creew menbers abourd) , which are then assembled when the accelerationfase is over . The re-asemby would be a very difficult process , innvolving a lot of high-level dicisions and unexpected problems , just the stuff humans are best at .’

If the weight is distributed on the sail they can be pulled together via cables after the acceleration phase. I think the energy requirement would be more because we must limit the acceleration to human tolerance levels where as AI’s can handle 10 of thousands of G’s reducing the beam divergence effects..

ljk : ”Perhaps if more folks studied the people who have lived in the polar regions of Earth and what prolonged isolation and constant danger have done to many of them, they will not be so cavalier when it comes time to send our children into the void.”
Perhaps if more people studied the ESKIMOES , who have fitted your job-description perfectly for 10,000 years , they would not be so pessimistic about what humans can do when they live in a TRIBE .. also it would seem likely that the sad cases you were talking about could be the result of an underdeveloped selectionprocess .

Previous posters in addition to the article’s author have already pointed out flaws in the book. I would like to add that the author writes 400 years in the future, but does not have a balanced view of what life and specifically science and engineering would be like in those days. Instead, it is like a combination of today’s science (in all fields) plus some Ship gizmos, like fusion engines. Also, there would have been plenty of time to resolve all the engineering issues within the solar system and not send the ship out without resolving them. Kudos to the article’s authors for their excellent research.

@ole burde: If by “self-replicating machines” you meant “human level AI”, then you were correct about Hawking and Musk. It would help, though, to use the correct term, because the two have completely different meanings.

1) ” They most be able to control and maintain the whole super-complex system autonomously, and deal with any unexpected problems.”
With this one you might as well have been speaking of a bacterium, animal, or other non-intelligent life-form, all of whom manage to maintain their own super-complex systems just fine. Clearly, no human level intelligence is required for this aspect of the task.

2) “Later they most be capable of raising the first generation of humans , without causing the children too many psycologic problems”
This one is, of course, not nearly as clear. There are many instances in fiction and folklore of children growing up in a human-free environment: Romulus and Remus, Tarzan, and Mowgli from the Jungle Book are famous examples. Off the top of my head, I do not know of any documented real examples, and obviously experiments cannot be performed. However, children are generally incredibly resilient. As a parent myself, I tend to be of the opinion that they will grow up just fine if they are surrounded by (non-intelligent) machines that cater to every physical need, provide interactive tutorials in all conceivable subjects of human knowledge, and offer a huge library of human cultural artifacts like books and movies.

Of course, such a mission would amount to exactly the kind of unethical experiment that we are lacking, and we would have to think very hard about whether the ends can justify the means.

Michael : I was imagining a scenario where components of the starship where accelerated separatedly , separated by ”short” time intervals (the time it would to accelerate each unit ) . Each unit would have its own sail , and they would somehow navigate to meet up at a calculated point . Perhaps the first units would be slightly more heavy . This could reduce the demands for laser power-output and energy supply by a factor of perhabs a hundred ….

“Perhaps if more folks studied the people who have lived in the polar regions of Earth and what prolonged isolation and constant danger have done to many of them, they will not be so cavalier when it comes time to send our children ”
People lived and endured far worse conditions.From gulags in polar regions to Siberian villages isolated for decades.Compared to which a generation ship is a nice cozy place.

All,
Lots of interesting discussion of embryo ships, 3D printers, artificial wombs etc. But I think we should consider the possibility that SETI will eventually succeed and we may contact another hi tech civilization. If that happens there is an even faster way to explore (not colonize!). We just send all the instructions for making printers, artificial wombs, genomes etc. This is Hoyle’s A for Andromeda scenario. In this case information is transmitted at c and the only delay is at the receiver as the machinery must be fabricated.What you would get are what I would call “guest explorers” not colonizers who wipe out the indigenous cultures as we have done many times in the past.

I also want to weigh in on the notion of sending faithful circuit diagrams of the human brain and expecting the recipient cyborgs or other vessels to then exhibit the spectrum of human behavior. As a neuroscientist I can tell you this is very silly at this point in time. It is pretty much like expecting a road map of Los Angeles to provide a deep knowledge of english.

The bottom line is we know nothing of the underlying molecular events that constitute memory, thought, personality etc. These are the hard problems of neuroscience and won’t be answered easily, no matter what groups like the European brain mapping consortium or Ray Kurzweil say. Sure, given enough time we will solve these problems but right now the prospects for cold sleep or even artificial wombs are better.
Joe

The harshest places on Earth are paradises compared to anywhere in space itself or any other world we know of. But you have such a faith that humanity will somehow manage no matter how bad things get during a long interstellar mission. Yes, they may survive physically even unaltered, but what will come out at the other end? You sure it will be something that you want to hail as human?

The point is, humans are designed and adapt to living on Earth, though for most of our existence that living has been more akin to surviving. Many of us only think we want a simpler life like our ancestors because we are often overwhelmed by our complex technological civilization, which in all honesty while we enjoy the comforts and the modern medicine of our society many of the other aspects have created huge problems for many people because quite frankly we have not really evolved past living in small tribes.

Maybe being a small tribe aboard a Worldship will actually work, so long as life is not made draconian (not harsh, note the difference) and then you will have to ensure that the future generations don’t go crazy breeding – and how do you control that? What if they want more children? Or none at all? Will you punish them? I hope there will be a better answer to this than “They’ll tough it out” or some such outmoded attitude. Otherwise I will again consider the thought that those are putting into designing a Worldship to not only be inadequate but dangerous and potentially deadly to the members of the vessel.

Still, I think either modified humans or outright Artilects will do a much better job exploring the settling the rest of the galaxy. Assuming any natives out there don’t get all tribal on them.

Eniac : ”..you might as well have been speaking of a bacterium, animal, or other non-intelligent life-form, all of whom manage to maintain their own super-complex systems just fine. Clearly, no human level intelligence is required for this aspect of the task.”
Thats a very good point . Biological systems apparently manage to perform super-complex control processes using only relatively simple feedback loop-structures ….but the reason they can do this sucsesfully , is that these structures have been selected by evolution after a billion years of trial-and -error testing . Inside our cells there are control structures which have been tested every second by evolution for at least 500 million years . It will not be easy to build a design procces which can do something similar , because any kind of simulation for testing will have to be identical to reality , otherwise reliability will go down the drain . That’s why complete automation of a comlicated product only pays off on the factory floor when you produce something in VERY big numbers ….which sadly will not be the case for our starship .

I just calculated the distance that the craft would move past Luyten 726-8 (A,B) and it is around 0.35 ly which is quite close and could potential move through it’s Oort cloud. But at ~0.12c I am not sure if the call that there is an iceberg ahead will get through in time to change course! Not sure either if the crafts trajectory could be bent by a close encounter with one or both stars towards Tau Ceti around 3.1 ly distance

@ole burde August 19, 2015 at 2:01

‘I was imagining a scenario where components of the starship where accelerated separatedly , separated by ”short” time intervals (the time it would to accelerate each unit ) . Each unit would have its own sail , and they would somehow navigate to meet up at a calculated point . Perhaps the first units would be slightly more heavy . This could reduce the demands for laser power-output and energy supply by a factor of perhabs a hundred ….’

The crewed section will always be the limiting factor because of the G limitation and the other components will have to catch up so we end up back to were we where. If we could use the sail material as a fuel and send them out ahead to be collected by the following craft it could reduce the fuel requirements greatly, possibly lithium6 dueteride which is a solid.

… but the reason they can do this sucsesfully , is that these structures have been selected by evolution after a billion years of trial-and -error testing

You say that as if it was an advantage. I think it is clear that intelligent design is many orders of magnitude more efficient at creating complex systems than evolution. Using intelligent design, we have outperformed nature in almost all things we set out to do, and done it in decades rather than billions of years. Aircraft fly faster and carry more load than any bird, cameras see better than any eye, transistors are faster and smaller than any neuron, solar cells are more efficient than leaves, etc. etc. It is hard to come up with many things that our designs do not do better than their natural equivalents, but intelligence and self-replication are indeed two of them. Which, in my opinion, simply means it will take another decade or two for us to catch up.

Michael :”The crewed section will always be the limiting factor because of the G limitation” … The real limiting factor will the cost of producing gigantic lacers with a gigantic energy supply which will both be used only for a very short time (perhaps only 12 days) , IF the ship is boosted in one section. If the ship is divided into a hundred sections boosted separately , the beam-units and their energy supply can be one hundred times smaller . Instead of working only for 12 days , they will work for 1200 days . This could be calculated for a very small differences in velocity , giving the crew lots of time to gather together and assemble the components …time is not a problem as the journey will take hundreds of years . The last component might catch-up only after several years .
12 days at 10 G is a first guess at what a healthy person could endure suspended in water , perhabs together with a druggs reducing metabolism to a minimum . This would give 5% of C

‘The real limiting factor will the cost of producing gigantic lacers with a gigantic energy supply which will both be used only for a very short time (perhaps only 12 days)’

This is unlikely as the laser would most likely be used for other probes and solar system activities.

‘If the ship is divided into a hundred sections boosted separately , the beam-units and their energy supply can be one hundred times smaller . Instead of working only for 12 days , they will work for 1200 days .’

The energy would be the same unless the sail is much larger for its weight than in a complete whole massed sail as the light intercept would be the same. The advantage would be in the smaller sails been more controllable.

’12 days at 10 G is a first guess at what a healthy person could endure suspended in water , perhabs together with a druggs reducing metabolism to a minimum .’

12 days in a water bath (suited) could be a tall order, remember although the body is supported against flattening the heart and any organs that can move will move, for instance the tongue would weight 12 times normal which could block the throat inhibiting breathing.

In this thread J. Jason Wentworth asked about a comparison with another generation-starship study, How We Will Reach the Stars by John Macvey (1969), originally published in 1965 under the title Journey to Alpha Centauri (page numbers from the 1969 edition). Having never heard of this writer, I felt compelled to look him up. The book is essentially popular-science non-fiction but including a sixty-page novella on a journey to Alpha C.
John Wishart Macvey was born in Kelso in 1923. He was a research chemist, working for ICI, but also an enthusiastic amateur astronomer and space buff. He wrote several popular science books. How We Will Reach the Stars is a pretty comprehensive survey of the prospects for starflight as understood then, with chapters on star types, the likely distribution of worlds of other stars, and the possibility of alien life.
Interstellar propulsion possibilities covered include nuclear fission and fusion drives, and ion drives (p40,42). The fusion exhaust velocity is quoted as 20,000 km/s, which is very similar to the exhaust velocity used by Robinson for his starship in Aurora. (Strictly speaking on p40 Macvey speaks of ‘velocities produced’ when discussing the nuclear drives, and ‘exhaust velocity’ on p43 discussing the ion drive; I have taken him to mean exhaust velocities throughout.)
Macvey’s novella dramatises a generation-starship mission, just as in Aurora. The Columbus and the Drake, two mighty starships, are launched from their construction base in Earth orbit on January 12, 2500. Each carrying 3000 crew (p182), the ships cruise to Alpha Centauri at 2% of lightspeed (c), taking 215 years, or eight generations, to reach the target (p181)
Macvey doesn’t describe his ships in detail in the fiction, but in the non-fiction sections (p48) he says a likely form for a starship would be a sphere, rotating for spin gravity at the equator (not unlike Gerard K. O’Neill’s designs a decade later), and with thrust applied by engines along the rotation axis. The engines aren’t specified save that the ‘Star Drive Units’ are ‘nuclear motors’ (p185). To reach 2% of c at one gravity would take about eight days; the Columbus’s acceleration seems to be gentler than that, taking some weeks (p187, p217).
Macvey is good on the sociology (chapter 6). 6000 crew ought to be enough for a genetically diverse founding population on the colony world. A couple of years in there is a rebellion; under a Draconian regime the leaders suffer the death openly (p192). And there are tight controls on reproduction, not only with upper limits but minimal replacement rates too. And the first generation of crew muse on the ethics of condemning generations to come to imprisonment: ‘We had a choice. They have none’ (p194).
After about 150 years disaster strikes, when one of the Columbus’s engines blows up (p214). Both ships slow to a halt to enable a joint repair project, and then continue on their way.
This is one of the rare technical implausibilities in the book. There is no need for the manoeuvre – the ships could surely rendezvous under cruise – and to slow down and start again is hugely expensive in fuel. The most energetic propulsion system mentioned in Macvey’s technical sections uses nuclear fusion to deliver an exhaust velocity of 20,000 km/s. Given Columbus’s cruise speed of 2% c , or 6000 km/s, the dreaded rocket equation tells us that with these numbers, to reach cruise from a standing start would require that the ship’s launch mass should include 26% fuel load, and to decelerate would require that the cruise mass should include another 26% fuel load. And if you stop midway and have to accelerate up to cruise again, the logic tells you that the delivered payload at Alpha Centauri would be a mere 30% of the launch weight from Earth. If you use a less energetic technology the numbers are even worse. So, given Macvey never discusses massive fuel loads, the unplanned mid-journey stationary rendezvous seems unlikely.
The two ships limp on to Alpha Centauri, where the crews are lucky enough to find a habitable planet (chapter 17) – and luck is the world. Macvey did not anticipate the ability to examine exoplanets and their atmospheres from the solar system; his colonists would have to be sent into the unknown. But he did speculate on the possibility of extraterrestrial life inimical to humans (p157) – just as befell Robinson’s explorers in Aurora.
In all the book, is a pleasant and engaging read, the science and the sociology still pretty much standing up. And it is an interesting comparison to Robinson’s novel, anticipating much of the ground Stan covers. I’m grateful to have come across Macvey’s enthusiasm and expertise, all these years later. As he says in his preface (p ix), ‘Is it wrong to dream?’

Brett
“I do think KSR is right about our goals potentially shifting by the time we have these capabilities. ”
I do think that our goals will indeed change in time, and colonization is to me a secondary goal of space exploration, exploration and research of other life I believe would be the primary goal.Something that Robinson avoids mentioning-he is in the book quite firmly against interstellar travel at all, rather than just colonization(or rather he doesn’t separate one from the other).
As I mentioned earlier, the statistical chances that life would be widespread in the Galaxy are engrained heavily in KSR book, since the first planet humans settle has life on it, and it is quite nearby.
But yes, scientific research and exploration would be the primary goal of interstellar civilization. Colonization in itself solved with the very techniques that made interstellar travel possible in the first place. Mind you, not all life threatening scenarios are avoidable without interstellar colonization-some form of it would be imperative, just not a wide scale.
Btw-by any chance, has anyone tried pointing KSR to these two discussions? If not I am actually thinking of doing so, he can be contacted by email I believe.

Michael : ”The energy would be the same unless the sail is much larger for its weight than in a complete whole massed sail as the light intercept would be the same. The advantage would be in the smaller sails been more controllable.”
The delivered ENERGY would be the same , but this energy would be produced over a much longer period of time , and so the laser delivering it could be much smaller …P=W/t

Peter Popov: “Has anyone seen an upper limit on the entry speed in aerobraking schemes, say in the upper atmosphere of a gas giant. Can some combination with magnetic fields removing material limitations?”

This is all about transferring the projectile’s kinetic energy elsewhere, usually a combination of radiation and environmental heating. The usual reentry “shield” reaches an equilibrium temperature where the radiation from the shield equals the rate of conversion of kinetic energy to heat. A magnetic field might do something useful (the atmosphere may be heated to plasma temperature), but still must accomplish the same energy transfer as a shield by radiation and environment heating.

“I remember in Stanislav Lem’s book “Fiasco” there was aerobraking from a fraction of the speed of light…”

Apart from the energy transfer problem (see above), that’s a lot of acceleration! It won’t be pretty.

Peter Popov : the only way I can imagine to increase the upper limit , is to somehow use the relatively bigger surface-area-to-volume ratio of smaller bodies : an ant falling from an airplane lands unhurt because of this relationship . Perhaps nannosize bodies encapsulated in the right way could use some variation of this effect to brake down from very big velocities , and this could be usefull if you wated to seed a planet with life . Accelerated by laser and with no need for braking , such a lightweight mission could actually almost be build with present technology …. This scenario becomes even more relevant if we DON’T find any life , after having identified a great number of exoplanets with the theoretical ability to sustain life .

The red dwarf binary pair UV Ceti (or Luyten 726-8) is located between Tau Ceti and Sol at 8.7 light years distance. I wonder if a gravitational slingshot maneuver at UV Ceti could be used to decelerate the Ship enough to enable the Oberth maneuvers at Sol? This would increase the duration of the return trip, but what’s the hurry considering the crew is in cryosleep?

‘The red dwarf binary pair UV Ceti (or Luyten 726-8) is located between Tau Ceti and Sol at 8.7 light years distance. I wonder if a gravitational slingshot maneuver at UV Ceti could be used to decelerate the Ship enough to enable the Oberth maneuvers at Sol? This would increase the duration of the return trip, but what’s the hurry considering the crew is in cryosleep?’

The sling shoot manoeuvre can only be used to rob momentum from a difference in velocity of say the Sun and the star in question. Luyten 726-8 has only a +29 km/s relative velocity, going away, not a lot I am afraid, it would not even be enough to bend it towards Tau Ceti by any appreciable degree at high velocity.

Below is a neat tool for looking at the night sky from various instruments, WISE being one.

Just zoom in to see Luyten 726-8 but it can’t be resolved. But here is an image from another site, below again.

Way out of my depth here, but I wondered if someone could explain further why the 28-step Oberth deceleration maneuver couldn’t work? I understand that one maneuver around the sun wouldn’t be enough, but in the book the ship/computer calculates their initial maneuver so as to aim itself at a planet for the second maneuver, and so on and so on. Is the reviewer saying that the second maneuver would be doomed because the ship is going to fast/the mass of Saturn (IIRC) is too small? If so that sounds plausible but I would be interested to see the math. By the way, thanks for providing the math on so many of these other “fact-checking” items–very educational!

The amount of ‘bend’ from an Oberth maneuver is determined by orbit — the entrance speed and how close to the body you get. Remember to go into an orbit of a body that you want to be less than the escape velocity. The reviewer states that the very first maneuver will still be at a 3% of the speed of light and there will be very little curvature at all. If the first maneuver is with the “Sun” at roughly the Sun’s outer boundary, then the escape velocity is 617.5 km/sec (0.002c). The ship will ‘bend’ its path only slightly and then shoot out of the solar system as an unbound projectile. You need to lose most of your speed before even coming close to a gravitational body which means engines and lasers — or run into a planetary astroid or two ;-)

Science fiction is full of ideas. But the ideas in science fiction seldom have the depth and rigor of ideas in science, or in philosophy, or politics and ethics. The reason I say this is: in fiction, the game is rigged. The debates are one-sided. The author gets the first, middle, and last word.

This is not to say that the ideas in science fiction cannot capture the imagination. Indeed many classic SF stories that have inspired careers or even presaged the future. But not all have. The ideas in SF are not fully developed theories or philosophies, but more like Edison’s famous ten thousand attempts at making electric light: we remember the one that worked and forget, mostly, the ten thousand that didn’t.

But the ones that work, either through vivid imagery or asking difficult questions or getting lucky and “predicting” the future, stay with us. Once in a great while, there is even a story that causes me to rethink my opposition to describing science fiction as a “literature of ideas.”

“The first problem I noticed was the staggering lack of curiosity evinced by the characters. You would think as arrival at Aurora approached, the two thousand colonists would be obsessed with finding out more about the new and alien world, and drafting scenarios to deal with any obvious problems. None of that happens. Moreover, this weird disinterest in the world around them isn’t limited to the latest generation. Until Devi made the effort, nobody had ever bothered to talk to the ship’s AI. WTF???”

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

If you'd like to submit a comment for possible publication on Centauri Dreams, I will be glad to consider it. The primary criterion is that comments contribute meaningfully to the debate. Among other criteria for selection: Comments must be on topic, directly related to the post in question, must use appropriate language, and must not be abusive to others. Civility counts. In addition, a valid email address is required for a comment to be considered. Centauri Dreams is emphatically not a soapbox for political or religious views submitted by individuals or organizations. A long form of the policy can be viewed on the Administrative page. The short form is this: If your comment is not on topic and respectful to your fellow readers, I'm probably not going to run it.